Withholding money due to me

Have been waiting and trying since FEBRUARY to have to LARGE Invoices which were processed incorrectly reversed and reprocessed because GEMS system messed up my Caeser authorisation. It is nearly July.... MANY CALLS EMAILS AND EMPYY PROMISED LATER I STILL DON'T HAVE MY MONEY. Interest adding up on my credit card and one doctor threatening to black list me because GEMS can't do their jobs properly. I don't know what else to do. I call and email.... And they promise to investigate... Then I get no reply. And We start again.